<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

</head>

<body>

  <div id="app">
    <button @click="sub">-</button>
    <span> {{num}} </span>
    <button @click="add">+</button>
    <div>
      <!-- 开发环境版本，包含了有帮助的命令行警告 -->
      <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
      <script>
        var app = new Vue({
          el: "#app",
          data: {
            num: 1
          },
          methods: {
            sub: function () {
              if (this.num > 0) {
                this.num--;
              } else {
                alert("别点啦,太小啦!");
              }
            },
            add: function () {
              if (this.num < 10) {
                this.num++;
              } else {
                alert("别点啦,太大啦!");
              }

            }

          }
        })
      </script>
</body>

</html>